Global Field Devices
COMMERCIAL HVAC COMPONENTS —
ENGINEERED FOR SUCCESS
Valve Selection and Sizing
Appendix A: Valve Selection and Sizing
Appendix A: Valve Selection and Sizing
Introduction
This section provides information on valve selection and sizing.
Valves must be selected for ability to meet temperature,
pressure, flow control characteristic, and piping connection
requirements of the hydronic system. Valve sizing is critical to
ensure support for heating and cooling loads with adequate
valve capacity, yet able to control system flow to provide stable
building conditions efficiently.
—
—
Definitions
Valve Components
Actuator:The part of an automatic control valve that moves the
stem based on an electric, electronic, or pneumatic
signal from a controller. The actuator and valve can be
two separate devices or together they can be one
device.
Body: The valve casting through which the controlled fluid
flows (Fig. 1).
STEM
DISC
HOLDER
DISC
SEAT
OUT
BODY
PLUG
M12225
Fig. 1. Globe Valve Components.
Bonnet: The part that screws to the top of the valve body and
contains the packing that seals and guides the valve
stem.
Disc:
Plug:
360
Port:
The opening in the valve seat.
Seat:
The stationary part of the valve body that has a raised
lip to contact the valve disc when closing off flow of the
controlled fluid.
Stem:
The shaft that runs through the valve bonnet and
connects an actuator to the valve plug.
Trim:
All parts of the valve that contact the controlled fluid.
Trim includes the stem, packing, plug, disc, and seat; it
does not include the valve body.
Valve Flow Characteristics
BONNET
IN
valve body. The shaped end controls fluid flow
through the valve with respect to stem travel.
A V-port plug has a cylinder, called a skirt, that
rides up and down in the valve seat ring. The skirt
guides the plug and varies the flow area with
respect to stem travel via its shaped openings.
A quick-opening plug is flat and is either endguided or guided by wings riding in the valve seat
ring. The flat plug provides maximum flow soon
after it lifts from the valve seat.
The part of the valve assembly that contacts the valve
seat to close off flow of the controlled fluid. Some valve
assemblies are built so the disc is replaceable.
Replaceable discs are usually made of a composition
material softer than metal. "Metal trim" valves use
precisely-machined metal plugs and seats operated by
high force actuators instead of a disk.
The part that varies the opening for the fluid to flow
through the valve body. The following describes the
three most common types of plugs:
— A contoured plug has a shaped end that is usually
end-guided at the top or bottom (or both) of the
Direction of Flow: The correct flow of the controlled fluid
through the valve is usually indicated on the valve
body. If the fluid flow through the valve is incorrect, the
disc can slam into the seat as it approaches the closed
position. The result is poor control, excessive valve
wear, and noisy operation. In addition, the actuator
must work harder to reopen the closed valve since it
must overcome the pressure exerted by the fluid on
top of the disc rather than have the fluid assist in
opening the valve by exerting pressure under the disc.
Gate and butterfly valves may offer bi-directional flow.
Equal percentage: A valve which changes flow by an equal
percentage (regardless of flow rate) for similar
movements in stem travel (at any point in the flow
range).
Linear: A valve which provides a flow-to-lift relationship that is
directly proportional. It provides equal flow changes for
equal lift changes, regardless of percentage of valve
opening.
Quick-opening: A valve which provides maximum possible
flow as soon as the stem lifts the disc from the valve
seat.
Valve flow characteristic: The relationship between the stem
travel of a valve, expressed in percent of travel, and
the fluid flow through the valve, expressed in percent
of full flow.
Appendix A: Valve Selection and Sizing
Valve Flow Terms
Rangeability: The ratio of maximum flow to minimum
controllable flow. Approximate rangeability ratios are
50 to 1 for V-port globe valves and 30 to 1 for
contoured plug valves.
EXAMPLE:
A valve with a total flow capacity of 100
gpm full open and a rangeability of 30 to 1,
can accurately controls flow accurately as
low as 3 gpm.
Tight shut-off/close-off: A valve condition in which virtually no
leakage of the controlled fluid occurs in the closed
position. Generally, only single-seated valves provide
tight shut-off. Double-seated valves typically have a
one to three percent leakage in the closed position.
Turndown: The ratio of maximum flow to minimum controllable
flow of a valve installed in a system. Turndown is equal
to or less than rangeability.
EXAMPLE:
For the valve in the rangeability example, if
the system requires a 66 gpm maximum
flow through the valve and since the
minimum accurately controllable flow is
3 gpm, the turndown is 22.
Valve Ratings
States Cv . The flow coefficients have the following
relationships:
Av
Av
=
0.0000278 kvs
=
0.0000240 Cv
kvs
=
0.865 Cv
The flow coefficient Av is in cubic meters per second
and can be determined from the formula:
ρ
A V = Q ------∆p
Where:
Q
=
ρ
∆p
=
=
volumetric flow in cubic meters per
second.
fluid density in kilograms per cubic meter.
static pressure loss across the valve in
pascals.
The flow coefficient kvs is water flow in cubic meters
per hour with a static pressure loss across the valve of
∆p
=
static pressure loss across the valve in
pascals.
The flow coefficient Cv is water flow in gallons per
minute with a pressure loss across the valve of one
pound per square inch within the temperature range of
40 to 100F and can be determined for other conditions
from the formula:
Where:
Q
=
ρ
=
ρw
=
∆p
=
1
ρ
C V = Q ------- • ------∆ρ ρ
w
volumetric flow in US gallons per minute.
fluid density in pounds per cubic foot.
density of water in pounds per cubic foot
within the temperature range of 40
to 100F
static pressure loss across the valve in
pounds per square inch.
Close-off rating: The maximum pressure drop that a valve can
withstand without leakage while in the full closed
position. The close-off rating is a function of actuator
power to hold the valve closed against pressure drop,
by structural parts such as the stem can be the limiting
factor. The construction of gate-style valves, such as
ball valves, often allows them to hold back high head
pressures in the closed position, although the actuator
may not be powerful enough to operate the valve
against such forces.
EXAMPLE:
A valve with a close-off rating of 10 psi
could have 40 psi upstream pressure and
30 psi downstream pressure. Note that in
applications where failure of the valve to
close is hazardous, the maximum
upstream pressure must not exceed the
valve close-off rating, regardless of the
downstream pressure.
The valve close-off rating is independent of the actual
valve body rating. See definition of BODY RATING
(ACTUAL).
361
APPENDICES
Flow coefficient (capacity index): Used to state the flow
capacity of a control valve for specified conditions. In
the control valve industry currently one of three flow
coefficients is used depending upon the location and
system of units; British Av, European kvs, or United
105 pascals (1 bar) within the temperature range of 5
to 40°C and can be determined from the formula:
∆ρk vs
ρ
k vs = Q ----------------- • ------∆ρ
ρw
Where:
Q
=
volumetric flow in cubic meters per hour.
ρ
=
fluid density in kilograms per cubic meter.
ρw
=
density of water in kilograms per cubic
meter.
∆p kvs=
static pressure loss of 105 pascals.
Appendix A: Valve Selection and Sizing
Close-off rating of three-way valves: The maximum pressure
difference between either of the two inlet ports and the
outlet port for mixing valves, or the pressure difference
between the inlet port and either of the two outlet ports
for diverting valves.
Pressure drop (critical): The flow of a gaseous controlled fluid
through the valve increases as the pressures drop
increases until reaching a critical point. This is the
critical pressure drop.
Any increase in pressure drop beyond the critical
pressure drop is dissipated as noise and cavitation
rather than increasing flow. The noise and cavitation
can destroy the valve and adjacent piping
components.
Body rating (nominal): The theoretical pressure rating,
expressed in psi, of the valve body exclusive of
packing, disc, etc. The nominal rating is often cast on
the valve body and provides a way to classify the valve
by pressure. A valve of specified body material and
nominal body rating often has characteristics such as
pressure-temperature ratings, wall thickness, and end
connections which are determined by a society such
as ANSI (American National Standards Institute).
Figure 2 shows ANSI pressure-temperature ratings for
valves. Note that the nominal body rating is not the
same as the actual body rating.
Body rating (actual): The correlation between safe, permissible
flowing fluid pressure and flowing fluid temperature of
the valve body (exclusive of the packing, disc, etc.).
The nominal valve body rating is the permissible
pressure at a specific temperature.
EXAMPLE:
From Figure 2, a valve with an ANSI rating
of 150 psi (ANSI Class 150) has an actual
rating of 225 psi at 250F.
362
ANSI
CLASS 250
300
ANSI
CLASS 150
250
LINE PRESSURE IN PSI
Pressure drop: The difference in upstream and downstream
pressures of the fluid flowing through the valve.
400
200
ANSI
CLASS 125
150
ANSI CLASS150
(STEAM)
100
212o F
50
0
0
275o F
50
100
150
200
250
300
FLUID TEMPERATURE IN oF
337o F
400
350
NOTES:
1. FOR HIGH FLUID TEMPERATURES, THE VALVE AND/OR
PIPING SHOULD BE INSULATED TO PREVENT AMBIENT
TEMPERATURES FROM EXCEEDING ACTURATOR RATINGS.
M12224
Fig. 2. Sample ANSI Pressure-Temperature Ratings
for Valves.
Maximum pressure and temperature: The maximum
pressure and temperature limitations of fluid flow that
a valve can withstand. These ratings may be due to
valve packing, body, or disc material or actuator
limitations. The actual valve body ratings are
exclusively for the valve body and the maximum
pressure and temperature ratings are for the complete
valve (body and trim). Note that the maximum
pressure and temperature ratings may be less that the
actual valve body ratings.
EXAMPLE:
The body of a valve, exclusive of packing,
disc, etc., has a pressure and temperature
rating of 125 psi at 335F. If the valve
contains a composition disc that can
withstand a temperature of only 240F, then
the temperature limit of the disc becomes
the maximum temperature rating for the
valve.
Appendix A: Valve Selection and Sizing
Valve Types
Ball valve:A ball valve has a precision ball between two seats
with a body (Fig. 3). Ball valves have several port sizes
for a give body size and go from closed to open with a
90 degree turn of the stem. They are available in both
two-way and three-way configurations. For HVAC
applications, ball valve construction includes brass
and cast iron bodies; stainless steel, chrome plated
brass, and cast iron balls; resilient seats with various
temperature ratings. Ball valves provide tight shut-off,
while full port models have low flow resistance, and
models with flow characterizing inserts can be
selected for modulating applications.
STEM
BODY
SEATS
BALL
PORT
Double-seated valve: A valve with two seats, plugs, and discs.
Double-seated valves are suitable for applications
where fluid pressure is too high to permit a single
seated valve to close. The discs in a double-seated
valve are arranged so that in the closed position there
is minimal fluid pressure forcing the stem toward the
open or closed position; the pressure on the discs is
essentially balanced. For a valve of given size and port
area, the double-seated valve requires less force to
operate than the single-seated valve so the double
seated valve can use a smaller actuator than a single
seated.
Also, double-seated valves often have a larger port
area for a given pipe size. A limitation of double-seated
valves is that they do not provide tight shut-off. Since
both discs rigidly connect together and changes in
fluid temperature can cause either the disc or the valve
body to expand or contract, one disc may seat before
the other and prevent the other disc from seating
tightly.
Flanged-end connections: A valve that connects to a pipe by
bolting a flange on the valve to a flange screwed onto
the pipe. Flanged connections are typically used on
large valves only.
M12228
Fig. 3. Ball Valve.
STEM
BODY
RESILIENT
SEAL
DISC
Gate valve: A valve that controls flow using a gating
mechanism, usually a plate, that moves across the
valve seat instead of pushing against the flow. The
actuator works against the friction of the seals rather
than directly against the force of the water. Gate valves
are inherently self-sealing and are often capable of
high close-off pressures without an actuator. Ball
valves are a type of gate valve.
Globe valve: A valve which controls flow by moving a circular
disk against or away from a seat. When used in
throttling control a contoured plug (throttling plug)
extends from the center of circular disk through the
center of the seat for precise control (Fig. 1).
Pressure-balanced valve: A globe valve with a sealed
pressure chamber built into the plug, which equalizes
head pressure across the seat and allows most of the
actuator force to be used to close off the flow, resulting
in very high close-off ratings with very low seat
leakage.
Reduced-port valve: A valve with a capacity less than the
maximum for the valve body. Ball, butterfly, and smaller
globe valves are available with reduced ports to allow
correct sizing for good control.
M12247
Fig. 4. Butterfly Valve.
363
APPENDICES
Butterfly valve: A valve with cylindrical bod, a shaft, and a
rotating disc (Fig. 4). The disc rotates 90 degrees from
open to closed. The disc seats against a resilient body
liner or spring-loaded metal seat and may be
manufactured for tight shut-off or made smaller for
reduced operating torque at lower close-off. Butterfly
valves have limited rangeability for modulating
applications so are used mainly for two-way operation.
For three-way applications, two butterfly valves are
assembled to a pipe tee with linkage for simultaneous
operation.
Appendix A: Valve Selection and Sizing
Single-seated valve: A valve with one seat, plug, and disc.
Single-seated valves are suitable for applications
requiring tight shut-off. Since a single-seated valve has
nothing to balance the force of the fluid pressure
exerted on the plug, it requires more closing force than
a double-seated valve of the same size and therefore
requires more actuator force than a double-seated
valve.
Petroleum products from sources such as cutting oils, solder
flux, etc. can cause some rubber compounds to swell and
interfere with moving parts.
Threaded-end connection: A valve with threaded pipe
connections. Valve threads are usually tapered female,
to National Pipe Thread standards, but male
connections are available for special applications.
Some valves have an integral union for easier
installation.
Particulate present in the system can interfere with, and
sometimes damage moving parts. Examples include: rust
(Fe2O3), magnetite (Fe3O4), sand (quartz granules), silt from
municipal water, iron filings from pipe threads, and scale
precipitated from hard water. Rust, in particular, is highly
abrasive and can rapidly wear out stem seals, causing leaks.
Three-way valve: A valve with three ports. The internal design
of a three-way valve classifies it as a mixing or
diverting valve. Three-way valves control liquid in
modulating or two-position applications and do not
provide tight shut-off.
To prevent damage to valves and pumps, a complete flushing
of the system during commissioning, including the existing
structure when building an addition, may be required to remove
physical particulate. Additional components may also be
needed, such as in-line Y-strainers for large objects such as
stones or solder blobs and mechanical filtration, such as a 50
micron 10% side-stream filter piped in parallel with the system
pumps.
Two-way valve: A valve with one inlet port and one outlet port.
Two-way valves control water or steam in two-position
or modulating applications and provide tight shut-off in
both straight through and angle patterns.
Valve Material and Media
Valves with bronze or cast iron bodies having brass or stainless
steel trim perform satisfactorily in HVAC hydronic systems when
the water is treated properly. Failure of valves in these systems
may be an indication of inadequate water treatment. The
untreated water may contain dissolved minerals (e.g., calcium,
magnesium, or iron compounds) or gases (e.g., carbon dioxide,
oxygen, or ammonia). Inadequate treatment results in corrosion
of the system. Depending on the material of the valve, the color
of the corrosion may indicate the substance causing the failure
(Table 1).
Table 1. Corrosive Elements in Hydronic Systems.
Brass or Bronze Component
Corrosive Substance
Corrosion Color
Light Blue-Green
Chloride
Blue or Dark Blue
Ammonia
Dark Blue-Green
Carbonates
White
Magnesium or Calcium
Black (water)
Oxides
Black (Gas)
Sulphide (Hydrogen)
Rust
Iron
Iron or Steel Component
Corrosive Substance
Corrosion Color
Magnesium or Calcium
White
Iron
Rust
364
Chloramines, chemical compounds of ammonia and chlorine
used to treat municipal drinking water, are reported to attack
some rubber compounds commonly used in closed loop
hydronic systems.
Glycol solutions may be used to prevent hydronic systems
freezing. Glycol solutions should be formulated for HVAC
systems. Some available glycol solutions formulated for other
uses contain additives that are injurious to some system seals.
In addition, hydronic seals react differently to water and glycol
such that when a new system is started up with water or glycol
the seals are effective. The hydronic seals are likely to leak if
the system is later restarted with media changed from to water
to glycol or glycol to water. To prevent leakage part of the
process of media changeover should include replacing seals
such as, pump and valve packing. Glycol mixtures are usually
limited to 50% concentration. At 60% concentration, glycol
mixtures have their minimum freezing temperature, but can
have unstable phase changes which may severely damage a
system.
Appendix A: Valve Selection and Sizing
Valve Selection
Proper valve selection matches a valve to the control and
hydronic system physical requirements. First consider the
application requirements and then consider the valve
characteristics necessary to meet those requirements. The
following questions provide a guide to correct valve selection.
— What is the piping arrangement and size?
The piping arrangement indicates whether a two-way or
three-way mixing or diverting valve is needed. The piping
size gives some indication of whether the valve requires a
screwed end or a flanged end connection.
— Does the application require two-position control or
proportional control? Does the application require a normally
open or normally closed valve? Should the actuator be direct
acting or reverse acting?
In its state of rest, the valve is normally open or closed
depending on the load being controlled, the fluid being
controlled, and the system configuration.
— What type of medium is being controlled? What are the
temperature and pressure ranges of the medium?
Valves must be compatible with system media composition,
maximum and minimum temperature, and maximum
pressure. The temperature and pressure of the medium
being controlled should not exceed the maximum
temperature and pressure ratings of the valve.
For applications such as chlorinated water or brine, select
valve materials to avoid corrosion.
— What is the pressure drop across the valve? Is the pressure
drop high enough?
The full open pressure drop across the valve must be high
enough to allow the valve to exercise control over its portion
of the hydronic system. However, the full open pressure drop
must not exceed the valves rating for quiet service and
normal life. Closed pressure drop must not exceed valve and
actuator close-off rating.
Globe Valve
For chilled water coils, it is usually preferable to close the
valve on fan shutdown to prevent excessive condensation
around the duct and coil, and to save pumping energy. This
may be accomplished with either normally closed valves or a
variety of other control schemes. Lower cost and more
powerful normally open valve assemblies may be used with
the close-on-shutdown feature and allow, in the case of
pneumatic systems, the capability to provide heating or
cooling in the event of air compressor failure.
— Is tight shut-off necessary? What differential pressure does
the valve have to close against? How much actuator closeoff force is required?
Valves should never be allowed to "dead head" a pump
unless the pumps are controlled by variable speed drive
systems capable of detecting such conditions and shutting
down the pumps.
A two-way globe valve has one inlet port and one outlet port
(Fig. 5) in either a straight through or angle pattern. The valve
can be either push-down-to-close or push-down-to-open.
Pneumatic and electric actuators with linear motion to operate
globe valves are available for operation with many control
signals.
Single-seated valves provide tight shut-off, while doubleseated valves do not. Double seated valves are acceptable
for use in pressure bypass or in-line throttling applications.
IN
The design and flow capacity of a valve determine who much
actuator force is required for a given close-off. Therefore, the
valve must first be sized, then, the valve and actuator
selected to provide the required close-off.
IN
PUSH-DOWN-TO-CLOSE
(DIRECT ACTING)
PUSH-DOWN-TO-OPEN
(REVERSE ACTING)
C2328A
Fig. 5. Two-Way Globe Valves.
365
APPENDICES
Converter control valves should be normally closed and
outdoor air preheat valves should be normally open.
Globe valves are popular for HVAC applications. They are
available in pipe sizes from 1/2 in. to 12 in. and in a large variety
of capacities, flow characteristics, and temperature and
pressure capabilities. They provide wide rangeability and tight
shutoff for excellent control over a broad range of conditions.
Globe valves are made in two-way, straight or angle
configurations and three-way mixing and diverting designs.
Globe valves close against the flow and have arrows on the
body indicating correct flow direction. Incorrect piping can result
in stem oscillations, noise, and high wear.
Appendix A: Valve Selection and Sizing
Ball Valve
Ball valves are available for two-position applications either
manual (hand) or power operated or for modulating applications
with direct coupled electric actuators. Ball valves are relatively
low cost, provide tight close off, and are available in two-way
and three-way configurations. As with all other valves, ball
valves must be properly sized to provide good flow control.
When butterfly valves are used for proportional control, they
must be applied using conservative pressure drop criteria. If the
pressure drop approaches the critical pressure drop,
unbalanced forces on the disc can cause oscillations, poor
control, and/or damage to the linkage and actuator, even
though the critical flow point is not reached. Modulating control
is usually limited to a range of 15 to 65 degrees of disk rotation.
When used in modulating service, ball valves must be
specifically designed for modulating service as compared to
two-position service. Packing must provide leak-free sealing
through thousands of cycles to ensure trouble-free HVAC
service. The ball, stem and seals should be made of materials
that minimizes sticking and breakaway torque to achieve
smooth operation.
Butterfly valves are usually found in larger pipe sizes. For
example, two butterfly valves could be piped in a mixing
application to control the temperature of the water going back to
the condenser. The valves proportion the amount of tower water
and condenser water return that is flowing in the condenser
water supply line.
Two-way ball valves have equal percentage flow control
characteristics and flow in full-port models can be in either
direction.
Two-way valves are available as globe, ball, or butterfly valves.
The combination of valve body and actuator (called valve
assembly) determines the valve stem position. Two-way valves
control steam or water in two-position or proportional
applications (Fig. 7). They provide tight shutoff and are
available with quick-opening, linear, or equal percentage flow
characteristics. Control valves are typically installed on the
supply side of convectors and radiators, and the return side of
small-bore water coils used in fan-forced equipment.
Three-way ball valves can be used in either mixing or diverting
service. Full port models have linear flow control characteristics
for constant total flow. A popular option with 3-way valves is a
20% flow capacity reduction in the B port to equalize pressure
losses in a coil-bypass application.
Two-way Valve
Butterfly Valve
Butterfly valves (Fig. 6) control the flow of hot, chilled, or
condenser water in two-position or proportional applications.
Butterfly valves are available in two-way or three-way
configurations. Tight shutoff may be achieved by proper
selection of actuator force and body lining. The three-way valve
can be used in mixing or diverting applications with the flow in
any direction. The three-way valve consists of two butterfly
valves that mount on a flanged cast iron tee and are linked to an
actuator which opens one valve as it closes the other. Minimum
combined capacity of both valves occurs at the half-open
position.
M10403
Fig. 6. Butterfly Valve.
366
SUPPLY
TWO–WAY
VALVE
LOAD
RETURN
C2329
Fig. 7. Two-Way Valve Application.
Ideally, a control system has a linear response over its entire
operating range. The sensitivity of the control to a change in
temperature is then constant throughout the entire control
range. For example, a small increase in temperature provides a
small increase in cooling. A nonlinear system has varying
sensitivity. For example, a small increase in temperature can
provide a large increase in cooling in one part of the operating
range and a small increase in another part of the operating
range. To achieve linear control, the combined system
performance of the actuator, control valve, and load must be
linear. If the system is linear, a linear control valve is appropriate
(Fig. 8). If the system is not linear, a nonlinear control valve,
such as an equal percentage valve, is appropriate to balance
the system so that resultant performance is linear.
Appendix A: Valve Selection and Sizing
Linear Valve
NONLINEAR SYSTEM
RESPONSE
PERCENTAGE OF
FULL COOLING
100%
A linear valve may include a V-port plug or a contoured plug.
This type of valve is used for proportional control of steam or
chilled water, or in applications that do not have wide load
variations. Typically in steam or chilled water applications,
changes in flow through the load (e.g., heat exchanger, coil)
cause proportional changes in heat output. For example, Figure
10 shows the relationships between heat output, flow, and stem
travel given a steam heat exchanger and a linear valve as
follows:
— Graph A shows the linear relationship between heat output
and flow for the steam heat exchanger. Changes in heat
output vary directly with changes in the fluid flow.
RESULTANT
LINEAR SYSTEM
CONTROL
EQUAL PERCENTAGE
CONTROL VALVE
0%
TEMPERATURE
100%
C2330
Fig. 8. Linear vs. Nonlinear System Control.
QUICK-OPENING VALVE
A quick-opening two-way valve includes only a disc guide and a
flat or quick-opening plug. This type of valve is used for two
position control of steam. The pressure drop for a quick opening
two-way valve should be 10 to 20 percent of the piping system
pressure differential, leaving the other 80 to 90 percent for the
load and piping connections.Figure 9 shows the relationship of
flow versus stem travel for a quick-opening valve. To achieve 90
percent flow, the stem must open only 20 percent. Linear or
equal percentage valves can be used in lieu of quick-opening
valves in two-position control applications as the only significant
positions are full open and full closed.
— Graph B shows the linear relationship between flow and
stem travel for the linear control valve. Changes in stem
travel vary directly with changes in the fluid flow.
NOTE: As a linear valve just starts to open, a minimum
flow occurs due to clearances required to prevent sticking of the valve. Some valves have a
modified linear characteristic to reduce this minimum controllable flow. This modified characteristic is similar to an equal percentage valve
characteristic for the first 5 to 10 percent of stem
lift and then follows a linear valve characteristic
for the remainder of the stem travel.
100%
90%
— Graph C shows the linear relationship between heat output
and stem travel for the combined heat exchanger and linear
valve. Changes in heat output are directly proportional to
changes in the stem travel.
FLOW
QUICK-OPENING
CONTROL VALVE
20%
STEM TRAVEL
100%
APPENDICES
0%
Thus a linear valve is used in linear applications to provide
linear control.
C2331
Fig. 9. Flow vs. Stem Travel Characteristic of a QuickOpening Valve.
90%
100%
90%
FLOW
HEAT OUTPUT
100%
90%
HEAT OUTPUT
100%
20%
0%
20%
20%
20%
FLOW
GRAPH A
90% 100%
0%
20%
STEM TRAVEL
GRAPH B
90% 100%
0%
20%
STEM TRAVEL
GRAPH C
90% 100%
C2332
Fig. 10. Heat Output, Flow, and Stem Travel Characteristics of a Linear Valve.
EQUAL PERCENTAGE VALVE
An equal percentage valve includes a contoured plug or
contoured V-port shaped so that similar movements in stem
travel at any point in the flow range change the existing flow an
equal percentage, regardless of flow rate. In mathematical
terms, this is an exponential response.
367
Appendix A: Valve Selection and Sizing
EXAMPLE:
When a valve with the stem at 30 percent of its total lift and
existing flow of 3.9 gpm (Table 2) opens an additional 10 percent of its full travel, the flow measures 6.2 gpm or increases
60 percent. If the valve opens an additional 10 percent so
the stem is at 50 percent of its full travel, the flow increases
another 60 percent and is 9.9 gpm.
Table 2. Stem Position vs. Flow for Equal Percentage Valve.
Stem
Change
—
10% increase
10% increase
Position
30% open
40% open
50% open
Rate
3.9 gpm
6.2 gpm
9.9 gpm
Flow
Change
—
60% increase
60% increase
An equal percentage valve is used for proportional control in hot
water applications and is useful in control applications where
wide load variations can occur. Typically in hot water
applications, large reductions in flow through the load (e.g., coil)
cause small reductions in heat output. An equal percentage
valve is used in these applications to achieve linear control. For
example, Figure 11 shows the heat output, flow, and stem travel
relationships for a hot water coil, with 200F, entering water and
50F entering air and an equal percentage valve, as follows:
— Graph B shows the nonlinear relationship between flow and
stem travel for the equal percentage control valve. To reduce
the flow 50 percent, the stem must close 10 percent. If the
stem closes 50 percent, the flow reduces 90 percent.
— Graph C shows the relationship between heat output and
stem travel for the combined coil and equal percentage
valve. The combined relationship is close to linear. A 10
percent reduction in heat output requires the stem to close
10 percent, a 50 percent reduction in heat output requires
the stem to close 50 percent, and a 90 percent reduction in
heat output requires the stem to close 90 percent.
The equal percentage valve compensates for the
characteristics of a hot water application to provide a control
that is close to linear.
50%
100%
90%
HEAT OUTPUT
100%
FLOW
HEAT OUTPUT
100%
90%
— Graph A shows the nonlinear relationship between heat
output and flow for the hot water coil. A 50 percent reduction
in flow causes a 10 percent reduction in heat output. To
reduce the heat output by 50 percent, the flow must
decrease 90 percent.
50%
10%
0% 10%
50%
FLOW
100%
0%
50%
10%
50%
STEM TRAVEL
GRAPH A
90% 100%
0% 10%
GRAPH B
50%
STEM TRAVEL
GRAPH C
90% 100%
C2333
Fig. 11. Heat Output, Flow, and Stem Travel Characteristics of an Equal Percentage Valve.
Three-way Valves
Three-way valves (Fig. 12) control the flow of liquids in mixing or
diverting valve applications (Fig. 13). The internal design of a
three-way globe valve enables it to seat against the flow of
liquid in the different applications. An arrow cast on the valve
body indicates the proper direction of liquid flow. It is important
to connect three-way valve piping correctly or oscillations,
noise, and excessive valve wear can result. Three-way valves
are typically have linear flow characteristics, although, some are
equal percentage for flow through the coil with linear flow
characteristics for flow through the coil bypass. Ball valves are
also available in a three-way configuration, while two butterfly
valves can be made to act as a three-way valve.
MIXING
VALVE
IN
DIVERTING
VALVE
OUT
I
N
IN
OUT
O
U
T
Fig. 12. Three-Way Valves.
368
C2334A
Appendix A: Valve Selection and Sizing
DIVERTING VALVE
THREE-WAY
MIXING VALVE
HOT
WATER
SUPPLY
LOAD
BYPASS
HOT
WATER
RETURN
A. LOAD BYPASS IN MIXING VALVE APPLICATION
THREE-WAY
DIVERTING VALVE
SUPPLY
LOAD
RETURN
BYPASS
B. LOAD BYPASS IN DIVERTING VALVE APPLICATION
C2335A
Fig. 13. Three-Way Valve Applications.
MIXING VALVE
A mixing valve provides two inlet ports and one common outlet
port. The valve receives liquids to be mixed from the inlet ports
and discharges the liquid through the outlet port (Fig. 12). The
position of the valve disc determines the mixing proportions of
the liquids from the inlet ports.
The close-off pressure in a mixing valve equals the maximum
value of the greater inlet pressure minus the minimum value of
the downstream pressure.
In globe mixing valve applications, the force exerted on the
valve disc due to unbalanced pressure at the inlets usually
remains in the same direction. In cases where there is a
reversal of force, the force changes direction and holds the
valve disc off the seat, cushioning it as it closes. If the pressure
difference for the system is greater than the pressure ratings of
available globe mixing valves, use a ball mixing valve or two
butterfly valves in a tee configuration.
The close-off pressure in a diverting valve equals the maximum
value of the inlet pressure minus the minimum value of the
downstream pressure.
Globe diverting valves must not be used for mixing service. As
with mixing valves used for diverting service, media pressure
drop across the valve can cause it to slam shut with resulting
loss of control.
EXAMPLE:
A diverting valve application has 20 psi maximum on the inlet
port, one outlet port discharging to the atmosphere, and the
other outlet port connecting to a tank under 10 psi constant
pressure. The pressure difference between the inlet and the
first outlet port is 20 psi and between the inlet and second
outlet port is 10 psi. The application requires a diverting
valve with at least 20 psi close-off rating.
Valve Sizing
Every valve has a capacity index or flow coefficient (Cv).
Typically determined for the globe and ball valves at full open
and about 60 degrees open for butterfly valves. Cv is the
quantity of water in gpm at 60F that flows through a valve with a
pressure differential of 1 psi. Sizing a valve requires knowing
the medium (liquid or gas) and the required pressure differential
to calculate the required Cv. When the required Cv is not
available in a standard valve, select the next closest and
calculate the resulting valve pressure differential at the required
flow to verify to verify acceptable performance.
After determination of the valve Cv, calculation of the flow of any
medium through that valve can be found if the characteristics of
the medium and the pressure drop across the valve are known.
Globe mixing valves are not suitable for modulating diverting
valve applications. If a mixing valve is piped for modulating
diverting service, the inlet pressure slams the disc against the
seat when it nears the closed position. This results in loss of
control, oscillations, and excessive valve wear and noise.
Mixing valves are acceptable using about 80 percent of the
close-off rating, but not recommended, in two-position diverting
valve applications.
369
APPENDICES
EXAMPLE:
A mixing valve application has a maximum pressure of
25 psi on one inlet port, maximum pressure of 20 psi on the
other inlet port, and minimum downstream pressure of
10 psi on the outlet port. The close-off pressure is 25 psi –
10 psi = 15 psi. The application requires a mixing valve with
at least a 15 psi close-off rating. The actuator selected must
have a high enough force to operate satisfactorily.
A globe diverting valve provides one common inlet port and two
outlet ports. The diverting valve uses two V-port plugs which
seat in opposite directions and against the common inlet flow.
The valve receives a liquid from one inlet port and discharges
the liquids through the outlet ports (Fig. 12) depending on the
position of the valve disc. If the valve disc is against the bottom
seat (stem up), all the liquid discharges through the side outlet
port. If the valve disc is against the top seat (stem down), all the
liquid discharges through the bottom outlet port.
Appendix A: Valve Selection and Sizing
Water Valves
Determine the capacity index (Cv) for a valve used in a water
application, using the formula:
REPRINTED BY PERMISSION FROM ASHRAE HANDBOOK—
1996 HVAC SYSTEMS AND EQUIPMENT
Fig. 14. Pressure Drop Correction for
Ethylene Glycol Solutions
Q G
C V = ------------h
G
h
Flow of fluid in gallons per minute required
to pass through the valve.
Specific gravity of the fluid (water = 1).
Pressure drop in psi. See Figures 14 and
15 for glycol solution correction values.
=
=
NOTE: The calculated Cv will rarely match the Cv of an available valve. For most accurate proportional control,
select the valve with the next lower Cv value, and
increase the pressure drop across the control valve to
achieve the required flow through the coil by reducing
the setting of the balancing valve. Otherwise, turndown ratio will be reduced, proportionally.
For example, if the calculated Cv is 87, and the two
closest Cv values are 63 and 100, the best choice for
control precision would be the valve with a Cv of 63,
and increase pressure drop across the valve by 90%.
If increased pressure drop is not possible, use the
valve with Cv of 100, and accept a 13% reduction in
valve rangeability.
PRESSURE DROP CORRECTION FACTOR
1.6
Where:
Q
=
40%
1.4
PROPYLENE
GLYCOL
SOLUTION
20%
1.2
10%
1.0
WATER
0.8
0
40
80
TEMPERATURE, F
PRESSURE DROP CORRECTION FACTOR
40%
50% BY MASS
Quantity of Water
To find the quantity of water (Q) in gallons per minute use one of
the following formulas:
1.
When Btu/hr is known:
Where:
Btu/hr=
K
=
TDw =
Btu ⁄ hr
Q = ---------------------K × TD w
Heat output.
Value from Table 3; based on temperature
of water entering the coil. The value is in
pounds per gallon x 60 minutes per hour.
Temperature difference of water entering
and leaving the coil.
Table 3. Water Flow Formula Table
Temp F
ETHYLENE
GLYCOL
SOLUTION
40
60
80
100
120
150
180
1.4
30%
1.2
20%
10%
1.0
WATER
0.8
0
370
40
80
TEMPERATURE, F
120
160
M12226
160
M12227
Fig. 15. Pressure Drop Correction for Propylene Glycol
Solutions.
Water
1.6
120
REPRINTED BY PERMISSION FROM ASHRAE HANDBOOK—
1996 HVAC SYSTEMS AND EQUIPMENT
For two-position control, always chose the largest Cv
greater than the coil with acceptable close-off pressure
rating.
Determining the Cv of a water valve requires knowing the
quantity of water (gpm) through the valve and the pressure drop
(h) across the valve. If the fluid is a glycol solution, use the
pressure drop multipliers from either Figure 14 or 15. See the
sections on QUANTITY OF WATER and WATER VALVE
PRESSURE DROP. Then select the appropriate valve based on
Cv, temperature range, action, body ratings, etc., per VALVE
SELECTION guidelines.
50% BY MASS
30%
Water
K
502
500
498
496
495
490
487
Temp F
200
225
250
275
300
350
400
K
484
483
479
478
473
470
465
Appendix A: Valve Selection and Sizing
2.
For hot water coil valves:
cfm × 1.08 × TD a
Q = --------------------------------------------K × TD w
Where:
cfm =
Airflow through the coil.
1.08 =
A scaling constant. See Note.
TDa =
Temperature difference of air entering and
K
=
TDw =
leaving the coil.
Value from Table 3; based on temperature
of water entering the coil (pounds per
gallon x 60 minutes per hour).
Temperature difference of water entering
and leaving the coil.
NOTE: The scaling constant 1.08 is derived as follows:
0.24BTU 60min 1lbair
1.08 = ----------------------- × ---------------- × -------------------lbairF
1hr
3
13.35ft
Where:
1lbair
-------------------- =
3
13.35ft
the specific volume of air at standard
conditions of temperature and
atmospheric pressure.
Simplifying the equation:
14, 40Btumin
1.08 = ----------------------------------3
Fhr13.35ft
To find the scaling constant for air conditions other
than standard, divide 14.40 Btu by specific volume of
air at those conditions.
For fan system chilled water coil valves:
Where:
cfm =
Btu/lb =
113 =
TDw =
cfm × Btu ⁄ lb
Q = -----------------------------------113 × TD w
Airflow through the coil.
Heat per pound of dry air removed.
Includes both sensible and latent heat.
A scaling constant.
Temperature difference of water entering
and leaving the coil.
To determine valve pressure drop:
1. For two-way valves consider the following guidelines for
valve pressure drop:
a. Include the pressure drop in the design of the water
circulating system.
— In systems with two-way valves only, it is often
necessary to provide a pump relief bypass or
some other means of differential pressure control
to limit valve pressure drops to the valve
capabilities. For control stability at light loads,
pressure drop across the fully closed valve should
not exceed triple the pressure drop used for sizing
the valve.
— To avoid high pressure drops near the pump,
reverse returns are recommended in large
systems.
b. The pressure drop across an open valve should be
about half of the pressure difference between system
supply and return, enough so that the valve, not the
friction through the coil or radiator, controls the volume of water flow or the valve pressure drop should
be equal to or greater than the pressure drop through
the coil or radiator, plus the pipe and fittings connecting them to the supply and return mains.
c. Verify allowable full open and full closed pressure
drops for all proportional and two-position water
valves with appropriate manufacturer literature.
d. Make an analysis of the system at maximum and minimum rates of flow to determine whether or not the
pressure difference between the supply and return
mains stays within the limits that are acceptable from
the stand point of control stability and close-off rating.
2. For two- and three-way valves consider the following
guidelines for valve pressure drop:
a. In load bypass applications (Fig. 13) such as radiators, coils, and air conditioning units, the pressure
drop should be 50 to 70 percent of the minimum difference between the supply and return main pressure
at design operating conditions.
b. A manual balancing valve may be installed in the
bypass to equalize the load drop and the bypass drop.
3. When selecting pressure drops for three-way mixing
valves in boiler bypass applications (Fig. 13), consider the
following:
a. Determine the design pressure drop through the
boiler including all of the piping, valves, and fittings
from the bypass connection through the boiler and up
to the three-way valve input.
b. The valve pressure drop should be equal to or greater
than the drop through the boiler and the fittings. If the
valve drop is much smaller than the boiler pressure
drop at design, effective control is obtained only when
the disc is near one of the two seats. The mid-portion
of the valve lift will be relatively ineffective.
371
APPENDICES
3.
WATER VALVE PRESSURE DROP
Appendix A: Valve Selection and Sizing
c. A manual balancing valve may be installed in the
boiler bypass to equalize the boiler drop and the
bypass drop.
WATER VALVE SIZING EXAMPLES
EXAMPLE 1:
A two-way linear valve is needed to control flow of 45F
chilled water to a cooling coil. The coil manufacturer has
specified an eight-row coil having a water flow pressure drop
of 3.16 psi. Further, specifications say that the coil will produce 55F leaving air with a water flow of 14.6 gpm. Supply
main is maintained at 40 psig, return is at 30 psig. Select
required capacity index (Cv) of the valve.
Use the water valve Cv formula to determine capacity index
for Valve V1 as follows:
Q G
C v = ------------h
Where:
Q
=
Flow of fluid in gallons per minute required
is 14.6 gpm.
G
=
Specific gravity of water is 1.
h
=
Pressure drop across the valve. The
difference between the supply and return
is 10 psi. 50% to 70% x 10 psi = 5 to 7 psi.
Use 6 psi for the correct valve pressure
drop. Note that 6 psi is also greater than
the coil pressure drop of 3.16 psi.
Substituting:
14.6 1
----------------=
6
6
Select a linear valve providing close control with a capacity
index of 6 and meeting the required pressure and temperature
ratings.
=
Cv
EXAMPLE 2:
A bypass valve is required to prevent flow through the chiller
from dropping below 90 percent of design flow. When sizing
valves for pump or chiller bypass applications (Fig. 16), system conditions that cause the valve to open or close completely must be considered before a pressure drop can be
selected.
Assume the following:
System flow at design, 1000 gpm
Pump head at design, 48 ft
Pump head at 90 percent flow, 50 ft
Pressure across mains at AHU 1 at design flow, 28 ft
Chiller pressure drop, 12 ft
Chiller piping loop design pressure drop, 8 ft
With full system flow, Valve V5 is closed. Pressure drop
across V5 equals the pump head minus the friction drops to
V5. Pressure drop across Valve V5 is then 48 ft – 12 ft
(chiller drop) – 4 ft (supply drop) – 4 ft (return drop) or 28 ft.
With system flow at 90 percent, the pump head rises to
50 ft, while the friction drops fall to the lower values shown in
Figure 16. For additional information on chiller bypass operation see Chiller, Boiler, and Distribution System Applications
section. Pressure drop across V5 equals the pump head
minus the friction drops to V5. Pressure drop across Valve
V5 is then 50 ft – 9.6 ft (chiller drop) – 3.2 ft (supply drop) –
3.2 ft (return drop) or 34 ft. Converting ft to psi, 34 ft x 0.4335
psi/ft = 14.7 psi.
Substituting the flow of water, specific gravity of water, and
pressure drop in the Cv formula shows that the Valve V5
should have a Cv of 235.
=
Cv
372
900 1
--------------=
14.7
235
Appendix A: Valve Selection and Sizing
4
PD
3.2
36
40.4
32
37.2
SUPPLY
CHILLER
12
PD
9.6
HEAT/
COOL
COIL
1
DP SETPOINT = 34'
DP
V1
V5
PUMP
180 GPM
PER
AHU COIL
HEAT/
COOL
COIL
4
V3
V2
V4
48
50
B1
0
HEAT/
COOL
COIL
3
HEAT/
COOL
COIL
2
4
PD
3.2
ZERO REFERENCE
B3
B2
B4
RETURN
SYSTEM STRAINER
4
3.2
NUMBERS IN CIRCLES
PUMP INLET
TOP NUMBERS
BOTTOM NUMBER
PD
=
=
=
=
=
GAGE PRESSURES
ZERO FOR SIMPLICITY
FULL FLOW
90% FLOW
M10605
PRESSURE DROP
Fig. 16. Chiller Bypass Application.
CASE A: 50 PSI
CASE B: 62 PSI
VALVE VI
180F
HOT WATER
SUPPLY
30% PRESSURE DROP, Cv = 12
70% PRESSURE DROP, Cv = 5
HEATING
COIL
40 PSI
LOCAL
HOT WATER
PIPING
RETURN
2.2 PSI
DROP
20 GPM AT DESIGN,
C2339A
4.3 PSI DROP
FLOW AT CONSTANT
PRESSURE DROP
100%
0%
30%
PRESSURE
DROP
70%
PRESSURE
DROP
IDEAL EQUAL
PERCENTAGE
VALVE
CHARACTERISTIC
STEM TRAVEL
100%
C2340
Fig 18. Effect of Pressure Drop in Hot Water Valve Sizing.
EXAMPLE 4:
A three-way mixing valve is needed for a heat exchanger
application with a bypass line. Water flow is specified at the
rate of 70 gpm. Manufacturer data for the exchanger indicates a pressure drop of 1.41 ft of water through the
exchanger coils.
Fig. 17. Equal Percentage Valve Hot Water Application.
373
APPENDICES
EXAMPLE 3:
Sizing water valves for heating coils is especially critical. In
Figure 17, a valve with a Cv of 12 will have 30 percent of the
available pressure drop when full open, while a valve with a
Cv of 5 will have 70 percent of the available pressure drop.
As shown in Figure 18, the valve with 70 percent of the available pressure drop essentially provides the equal percentage water flow control, resulting in linear coil heat transfer
and stable temperature control. The valve with only 30 percent of the available pressure drop has a more linear flow
control which results in nonlinear coil heat transfer. See
EQUAL PERCENTAGE VALVE section for further information.
Appendix A: Valve Selection and Sizing
( 1 + 0.00075s )Q V
C v = -------------------------------------------------63.5 h
Use the water valve Cv formula to determine capacity index
for Valve V1 as follows:
Where:
Q
=
Q G
C v = ------------h
Where:
Q
=
G
h
=
=
Flow of fluid in gallons per minute required
to pass through the valve is 70 gpm.
Specific gravity of water is 1.
Pressure drop across the valve. Plans of
the heating system indicate three-inch
supply and return mains. From an elbow
equivalent table and pipe friction chart
found in the ASHRAE Handbook or other
reference manuals, the calculated
pressure drop through a three-inch tee
and the piping from the valve and the tee
to the exchanger is
0.09 psi. Heat exchanger pressure drop is
1.41 ft of water or 1.41 ft x 0.433 psi/ft =
0.61 psi. Total pressure drop from bypass
connection through the heat exchanger
and to the hot-water input of the three-way
valve is 0.61 + 0.09 or 0.70 psi.
Since the valve pressure drop (h) should be equal to or
greater than the drop through the heat exchanger and fittings, 0.70 psi is used as the valve pressure drop.
V
=
63.5 =
h
=
s
=
Determining the Cv for a steam valve requires knowing, the
quantity of steam (Q) through the valve, the pressure drop (h)
across the valve, and the degrees of superheat. See
QUANTITY OF STEAM and STEAM VALVE PRESSURE
DROP. Then select the appropriate valve based on Cv,
temperature range, action, body ratings, etc., per VALVE
SELECTION guidelines.
NOTE: When the superheat is 0F, then (1 + 0.00075s) equals
1 and may be ignored.
QUANTITY OF STEAM
For optimum control, a manual balancing valve is installed in
the bypass line to equalize the pressure drops in the
exchanger and bypass circuits.
To find the quantity of steam (Q) in pounds per hour use one of
the following formulas:
70 1
=
Cv
------------=
83.6 or 84
0.70
Substituting the flow of water, specific gravity of water, and
pressure drop in the Cv formula shows that the valve should
have a Cv of 83.6 or 84.
Btu ⁄ hr
Q = ------------------------------------------------1000Btu ⁄ lbsteam
Select a linear valve providing close control with a capacity
index of 84 and meeting the required pressure and temperature ratings.
Steam Valves
Calculate the required capacity index (Cv) for a valve used in a
steam application, using the formula:
1.
When Btu/hr (heat output) is known:
Where:
Btu/hr =
1000 Btu/lb=
2.
Heat output.
A scaling constant representing the
approximate heat of vaporization of
steam.
For sizing steam coil valves:
CFM × TD a × 1.08
Q = ------------------------------------------------1000Btu ⁄ lbsteam
Where:
cfm
=
TDa
=
1.08
=
1000 Btu/lb=
374
Quantity of steam in pounds per hour
required to pass through the valve.
Specific volume of steam, in cubic feet per
pound, at the average pressure in the
valve. For convenience Table 5 at the end
of the STEAM VALVES section lists the
square root of the specific volume of
steam for various steam pressures.
Therefore, use the value in this column of
the table as is; do not take its square root.
A scaling constant.
Pressure drop in psi.
Superheat in degrees F.
Cubic feet per minute (ft3/min) of air from
the fan.
Temperature difference of air entering and
leaving the coil.
A scaling constant. See NOTE.
A scaling constant representing the
approximate heat of vaporization of
steam.
Appendix A: Valve Selection and Sizing
NOTE: The scaling constant 1.08 is derived as follows:
0.24BTU 60min 1lbair
1.08 = ----------------------- × ---------------- × -------------------lbairF
1hr
3
13.35ft
Where:
1lbair
-------------------3
13.35ft
=
the specific volume of air at standard
conditions of temperature and
atmospheric pressure.
Simplifying the equation:
14.40Btumin
1.08 = ---------------------------------3
Fhr13.35ft
To find the scaling constant for air conditions other
than standard, divide 14.40 Btu by specific volume of
air at those conditions.
3. For sizing steam to hot water converter valves:
Q = gpm × TD × 0.49
w
Where:
gpm =
TDw =
0.49 =
Gallons per minute of water flow through
converter.
Temperature difference of water entering
and leaving the converter.
A scaling constant. This value is derived
as follows:
8.33lbwater 60min 1lbsteam
1Btu
0.49 = -------------------------------- × ---------------- × -------------------------- × -----------------------1gal
1hr
1000Btu lb water F
Simplifying the equation:
4. When sizing steam jet humidifier valves:
3
( W 1 – W 2 )lbmoisture
1
ft
60min
Q = ----------------------------------------------------------- × --------------------- × ---------- × ---------------lbair
3 min
hr
13.35ft
-------------------lbair
Where:
W1 =
Humidity ratio entering humidifier, pounds
of moisture per pound of dry air.
W2 =
Humidity ratio leaving humidifier, pounds
of moisture per pound of dry air.
3
13.35ft
-------------------- = The specific volume of air at standard
lbair
conditions of temperature and
atmospheric pressure.
3
ft
---------- =
min
5. When Equivalent Direct Radiation (EDR) is known:
Q = EDR ( Total ) × 0.24
Where:
EDR (Total)=Radiators are sized according to
Equivalent Direct Radiation (EDR). If
controlling several pieces of radiation
equipment with one valve, add the EDR
values for all pieces to obtain the total
EDR for the formula.
0.24 =
A scaling constant, lb steam/unit EDR.
See Table 4.
Table 4. Output of Radiators and Convectors.
Average Radiator of
Cast Iron Radiator Convector, Btu/
Convector
Temperature, Deg F
Btu/Hr/EDRa
Hr/EDRb
215
240
240
200
209
205
190
187
183
180
167
162
170
148
140
160
129
120
150
111
102
140
93
85
130
76
69
120
60
53
110
45
39
100
31
27
90
18
16
a At Room Termperature
b At 65 F Inlet Air Temperature
STEAM VALVE PRESSURE DROP
Proportional Applications
When specified, use that pressure drop (h) across the valve.
When not specified:
1. Calculate the pressure drop (h) across the valve for good
modulating control:
h = 80% x (Pm-Pr)
NOTE: For a zone valve in a system using radiator orifices use:
Cubic feet per minute (cfm) of air from the
fan.
60min
---------------- =
hr
( W 1 – W 2 )lbmoisture
Q = 4.49 ----------------------------------------------------------hr
A conversion factor.
h = (50 - 75)% x (Pm-Pr)
Where
Pm =
Pr
=
Pressure in supply main in psig or psia
(gage or absolute pressure).
Pressure in return in psig or psia. A
negative value if a vacuum return.
375
APPENDICES
0.49minlbsteam
0.49 = --------------------------------------------galhrF
Simplifying:
Appendix A: Valve Selection and Sizing
2.
Determine the critical pressure drop:
hcritical = 50% x Pma
Where:
Pma =
psia =
Pressure in supply main in psia (absolute
pressure)
psig + 14.7
Use the smaller value h or hcritical when calculating Cv.
Substituting this data in the formula:
Q
=
808.5 pounds per hour
h
=
The pressure drop across a valve in a
modulating application is:
h = 85% x (Pm-Pr)
Where:
Pm =
Pr
=
Two-Position Applications
Use line sized valves whenever possible. If the valve size must
be reduced, use:
h = 20% x (Pm-Pr)
Where
Pm =
Pr
=
Pressure in supply main in psig or psia
(gage or absolute pressure).
Pressure in return in psig or psia. A
negative value if a vacuum return.
STEAM VALVE SIZING EXAMPLES
EXAMPLE 1:
A two-way linear valve (V1) is needed to control high-pressure steam flow to a steam-to-water heat exchanger. An
industrial-type valve is specified. Steam pressure in the supply main is 80 psig with no superheat, pressure in return is
equal to atmospheric pressure, water flow is 82.5 gpm, and
the water temperature difference is 20F.
Use the steam valve Cv formula to determine capacity index
for Valve V1 as follows:
( 1 + 0.00075s )Q V
C v = -------------------------------------------------63.5 h
Where:
Q
=
The quantity of steam required to pass
through the valve is found using the
converter valve formula:
Q = gpm × TD w × 0.49
Where:
gpm =
TDw =
82.5 gpm water flow through exchanger
20F temperature difference
0.49 =
A scaling constant
Upstream pressure in supply main is 80
psig.
Pressure in return is atmospheric
pressure or 0 psig.
Substituting this data in the pressure drop formula:
h
=
0.80 x (80 – 0)
=
0.80 x 80
=
64 psi
The critical pressure drop is found using the following formula:
hcritical = 50% x (psig + 14.7 psi)
hcritical=
=
=
0.50 x (80 psig upstream + 14.7 psi)
0.50 x 94.7 psi
47.4 psi
The critical pressure drop (hcritical) of 47.4 psi is used in calculating Cv, since it is less than the pressure drop (h) of 64
psi. Always, use the smaller of the two calculated values.
V
=
Pavg =
Specific volume (V) of steam, in cubic
feetper pound at average pressure in
valve (Pavg):
h
Pm – --2
47.4
=
– ---------80
=
– 23.6
56.4psig
80
2
The specific volume of steam at 56.4 psig is 6.14 and
the square root is 2.48.
=
63.5 =
A scaling constant.
Substituting the quantity of steam, specific volume of steam,
and pressure drop in the Cv formula shows that the valve
should have a Cv of 4.6.
( 1 + 0.00075 × 0 ) × 808.5 × 2.48
C v = ------------------------------------------------------------------------------63.5 47.4
1745.6
= --------------------------- = 4.6
63.5 × 6.88
NOTE: If Pavg is rounded off to the nearest value in
Table 5 (60 psi), the calculated Cv is 4.5 a negligible difference.
376
Appendix A: Valve Selection and Sizing
Select a linear valve providing close control with a capacity
index of 4 and meeting the required pressure and temperature ratings.
NOTE: For steam valves downstream from pressure
reducing stations, the steam will be superheated
in most cases and must be considered.
EXAMPLE 2:
In Figure 19, a linear valve (V1) is needed for accurate flow
control of a steam coil that requires 750 pounds per hour of
steam. Upstream pressure in the supply main is 5 psig and
pressure in the return is 4 in. Hg vacuum minimum.
5 PSI
C2336
Fig. 19. Linear Valve Steam Application.
Use the steam valve Cv formula to determine capacity index
for Valve V1 as follows:
( 1 + 0.00075s )Q V
C v = -------------------------------------------------63.5 h
h
=
Pr
=
Quantity of steam required to pass
through the valve is 750 pounds per hour.
The pressure drop across a valve in a
modulating application is found using:
80% x (Pm – Pr)
Upstream pressure in supply main is 5
psig.
Pressure in return is 4 in. Hg vacuum.
NOTE: 1 in. Hg = 0.49 psi and 1 psi = 2.04 in. Hg.
Therefore,
4 in. Hg vacuum = –1.96 psig.
h
=
=
=
0.80 x [5 – (–1.96)]
0.80 x 6.96
5.6 psi
0.50 x (5 psig upstream + 14.7 psi)
0.50 x 19.7 psia
9.9 psi
The pressure drop (h) of 5.6 psi is used in calculating
the Cv, since it is less than the critical pressure drop
(hcritical) of 9.9 psi.
=
Pavg =
Specific volume (V) of steam, in cubic feet
per pound at average pressure in valve
(Pavg):
h
Pm – --2
5.6
– ------5=
– 2.8
2.2psig
5=
2
The specific volume of steam at 2.2 psig is 23.54 and
the square root is 4.85.
63.5 =
A scaling constant.
s
=
0
=
Substituting the quantity of steam, specific volume of steam,
and pressure drop in the Cv formula shows that Valve V1
should have a Cv of 24.17 or the next higher available value
(e.g., 25).
( 1 + 0.00075 × 0 ) × 750 × 4.85
C v = --------------------------------------------------------------------------63.5 5.6
3637.5
= --------------------------- = 24.17
63.5 × 2.37
NOTE: If Pavg is rounded off to the nearest value in
Table 5 (2 psi), the calculated Cv is 24.30.
Select a linear valve providing close control with a capacity
index of 25 and meeting the required pressure and temperature ratings.
EXAMPLE 3:
Figure 20 shows the importance of selecting an 80 percent
pressure drop for sizing the steam valve in Example 2. This
pressure drop (5.6 psi) approximates the linear valve characteristic. If only 30 percent of the available pressure drop is
used (0.30 x 6.96 psi = 2.10 psi or 2 psi), the valve Cv
becomes:
( 1 + 0.00075s )Q V
C v = -------------------------------------------------63.5 h
750 × 4.85
C v = ------------------------- = 40.5
63.5 2
377
APPENDICES
h
=
and:
Pm =
hcritical=
=
=
RETURN
30% PRESSURE DROP, Cv = 41
80% PRESSURE DROP, Cv = 25
Where:
Q
=
hcritical = 50% x (psig + 14.7 psi)
V
VALVE VI STEAM 1.96 PSI
COIL (VACUUM)
SUPPLY
The critical pressure drop is found using the following formula:
Appendix A: Valve Selection and Sizing
This larger valve (2 psi drop) has a steeper curve that is further away from the desired linear valve characteristic. See
LINEAR VALVE under VALVE SELECTION for more information.
VALVE OPENING/
STEAM FLOW
100%
0%
Cv = 25
Cv = 41
LINEAR VALVE
CHARACTERISTIC
STEM TRAVEL
100%
C2337
Fig. 20. Effect of Pressure Drop in Steam Valve Sizing.
Table 5. Properties of Saturated Steam.
Boiling
Point or
Steam
V
Specific
Temperature Volume (V), (For valve
sizing)
cu. ft/lb
(Deg F)
26.57
706.00
76.6
12.04
145.00
133.2
8.672
75.20
161.2
7.162
51.30
178.9
6.950
48.30
181.8
6.576
43.27
187.2
6.257
39.16
192.2
5.984
35.81
196.7
5.744
32.99
201.0
5.533
30.62
204.8
5.345
28.58
208.5
Maximum
Allowable
Pressure
Drop, psi.
0.23
1.2
2.4
3.7
3.9
4.4
4.9
5.4
5.9
6.4
6.9
Boiling
Point or
Steam
V
Specific
TempGage
Pressure, erature Volume (V), (For valve
cu. ft/lb
sizing)
(Deg F)
psig
5.175
26.79
212.0
0
5.020
25.20
215.3
1
4.876
23.78
218.5
2
4.751
22.57
221.5
3
4.626
21.40
224.4
4
4.518
20.41
227.1
5
4.410
19.45
229.8
6
4.317
18.64
232.3
7
4.225
17.85
234.8
8
4.142
17.16
237.1
9
Maximum
Allowable
Pressure
Drop, psi.
7.4
7.8
8.4
8.8
9.4
9.8
10.4
10.8
11.4
11.8
Vacuum,
Inches of
Mercury
29
25
20
15
14
12
10
8
6
4
2
378
Boiling
Point or
Steam
V
Specific
TempGage
Pressure, erature Volume (V), (For valve
(Deg F)
psig
sizing)
cu. ft/lb
4.061
16.49
239.4
10
3.987
15.90
241.6
11
3.918
15.35
243.7
12
3.724
13.87
249.8
15
3.464
12.00
258.8
20
3.251
10.57
266.8
25
3.076
9.463
274.0
30
2.93
8.56
280.6
35
2.797
7.826
286.7
40
2.685
7.209
292.4
45
2.585
6.682
297.7
50
2.496
6.232
302.6
55
2.416
5.836
307.3
60
2.343
5.491
311.8
65
2.276
5.182
316.0
70
2.216
4.912
320.0
75
2.159
4.662
323.9
80
2.108
4.445
327.6
85
2.059
4.239
331.2
90
2.015
4.060
334.6
95
1.972
3.888
337.9
100
1.896
3.595
344.1
110
1.827
3.337
350.0
120
1.766
3.12
355.2
130
1.710
2.923
360.9
140
1.657
2.746
366.2
150
1.613
2.602
370.6
160
1.569
2.462
375.5
170
1.531
2.345
379.6
180
1.495
2.234
383.9
190
1.461
2.134
387.8
200
1.385
1.918
397.4
225
1.320
1.742
406.0
250
1.263
1.595
414.2
275
1.213
1.472
421.8
300
1.128
1.272
435.6
350
1.058
1.120
448.1
400
0.999
0.998
459.5
450
0.949
0.900
470.0
500
0.904
0.818
479.7
550
0.865
0.749
488.8
600
0.831
0.690
497.3
650
0.799
0.639
505.4
700
0.744
0.554
520.3
800
0.699
0.488
533.9
900
0.659
0.435
546.3
1000
Maximum
Allowable
Pressure
Drop, psi.
12.4
12.8
13.4
14.8
17.4
19.8
22.4
24.8
27.4
29.8
32.4
34.8
37.4
39.8
42.4
44.8
47.4
49.8
52.4
54.8
57.4
62.3
67.4
72.3
77.4
82.3
87.4
92.3
97.4
102.3
107.4
119.8
132.4
145.0
157.4
182.4
207.4
232.4
257.4
282.4
307.4
332.4
357.4
407.4
457.4
507.4
Technology Comparison of Control Ball and Globe Valves
Attribute
Flow Characteristics
Control Ball Valve
Globe Valve
Quadratic (with characterization)
Equal percent to design temp.
Linear (full port)
Linear
Delayed opening / early close-off
Rangeability
(turn-down ratio)
Operating Differential
Pressure
Close-Off Differential
Pressure
Seat Leakage
Fixed minimum flow results
in
1. Low TDR at low Cv
2. High TDR at high Cv
Advantage
Reason
Globe
BAS controller expects
flow from valve at low
signal
Globe
Small sizes are the most
common applications and
need high TDR
——
20+ psid is not typical of
control valve applications
Continuous from start
50:1 = 2% steps (HON)
100:1 (Siemens claim)
Maximum 25:1 (JCI)
20 – 25 psid for characterized ports
(plate distortion)
20 – 25 psid for quiet operation
(cavitation at low flow)
High with low Torque actuators
(water pressure aids sealing)
Inversely proportional to Cv, and
proportional to actuator force
Capable of dead-heading pumps*
Pressure balanced is high
ANSI Class IV (< 0.01% Cv) @ A port
(Does not apply to B port without
seals)
ANSI Class III w/ small metal seats
ANSI Class IV with resilient seat and
larger metal seated valves
Control
Ball
Globe is comparable
in small sizes
PB more expensive
Control
Ball
Less leakage reduces
energy use with chilled
water
——
Long term performance
of ball valve in automatic
control unknown
Globe
Greater versatility. Equal %
flow available with globe
Resilient material on metal seat
Trim
(internal
construction)
Plated brass ball and stem
Rubber and Teflon stem O-rings
Stainless steel ball and stem
Brass plug on brass seat
Stainless steel plug on SS seat
Low pressure
——
High pressure
Cv Ratings
Multiple Cv’s per valve size
Multiple Cvs @ 1/2" size
Ball
Lower installed cost with
no loss of control capability
Line Size Piping
Line size piping with lower Cv
Reducers often needed > 1/2"
Ball
Lower installed cost with
no loss of control capability
Globe
Wider applications
Control
Ball
Easier to select. Piping
different mixing & diverting
Depends on application
Pipe Sizes
1/2" – 3" Threaded
1/2" – 3" Threaded ANSI 150
4" – 6" ANSI 125 Flanged
2-1/2" – 6" ANSI 125 and 250
Flanged
Combo mixing/diverting
Mixing models
B port seal required for tight closeoff
Diverting models
Low profile at large sizes
Large profile at large sizes
Control
Ball
Relatively large in 1/2" pipe
Small size in 1/2" pipe
Globe
Floating/2-position, modulating
Floating/2-position, modulating
——
Some pneumatic actuators
available
Large linear pneumatic installed
base
Globe
N/O or N/C by actuator position
N/O or N/C up to 3"
Stay-in-place
Stay-in-place
Control
Ball
Globe needs higher power
actuators
Valve Serviceability
Requires unions for valve access
VBN stems replaceable
In-line serviceable
Globe
B.V. must be removed
from piping
CE Preference
Growing with time
Well established
Globe
Familiar technology (habit)
Easier to select
Added cost for globe valve
3-Way Body Styles
Physical Size
Control Inputs
Fail Safe Operation
Contractor
Preference
Valve comes with actuator
Actuator selection separate
Control
Ball
Actuator Selection
Match valve and damper DCAs
Requires linkage for DCAs
Control
Ball
Depends on application
*Pumps require pressure cut-offs or supply-return differential pressure regulators to avoid pump seal damage and potential leakage that can result without flow-through. Unless used in
end-of-line-service, control valves do not need to close off against full pump head.
379
APPENDICES
Low pressure (full port only)
Steam Ratings